\documentclass{article}
\usepackage{booktabs}
\begin{document}
\begin{center}
	\begin{tabular}{llll}
		\toprule
		Type            & \multicolumn{2}{l}{Register} & Purpose           \\
		\midrule
		General Purpose & ax & ah                      & Arithmetic        \\
		                &    & al                      &                   \\
		\addlinespace % \cmidrule{2-4}
		                & bx & bh                      & Base              \\
		                &    & bl                      &                   \\
		\addlinespace
		                & cx & ch                      & Counter           \\
		                &    & cl                      &                   \\
		\addlinespace
		                & dx & dh                      & Data              \\
		                &    & dl                      &                   \\
		\cmidrule{2-4}
		                & \multicolumn{2}{l}{sp}       & Stack Pointer     \\
		                & \multicolumn{2}{l}{bp}       & Base Pointer      \\
		                & \multicolumn{2}{l}{si}       & Source Index      \\
		                & \multicolumn{2}{l}{di}       & Destination Index \\
		\midrule
		Segment         & \multicolumn{2}{l}{ss}       & Stack Segment     \\
		                & \multicolumn{2}{l}{cs}       & Code Segment      \\
		                & \multicolumn{2}{l}{ds}       & Data Segment      \\
		                & \multicolumn{2}{l}{es}       & Extra Segment     \\
		                & \multicolumn{2}{l}{fs}       & Extra Segment     \\
		                & \multicolumn{2}{l}{gs}       & Extra Segment     \\
		\bottomrule
	\end{tabular}
\end{center}
\end{document}
